Outliner - show only hidden
Press A until everything is selected. If you have anything selected before pressing A you will have to press it twice, otherwise only once. (This will select the hidden objects as well so don't worry about them for now.)
Edit: As Mr. Zak pointed out in the comments section, it is unnecessary to select everything.

Press Alt-H to unhide everything.

You can also navigate to the Object menu at the bottom of the View3D window, select Show / Hide and select Show Hidden. This may be helpful if, like OP, you have issues with the Alt key combination. Practically everything in Blender has a button somewhere.